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09-22-2010, 08:57 AM   #16
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Just replace ETTL with PTTL (I use this on a Canon forum)

IF the flash is providing all the illumination (which it generally is in a small-ish room with you bouncing it off the ceiling), the shutter speed AND how dark it is do NOT matter AT ALL.

Try this - Pick a room in your house at night. Have a bunch of lights on. Set the shutter speed to 1/200s, aperture to f/5.6, ISO400. Point the flash straight up towards the ceiling (make sure the flash is in ETTL). Shoot.

Then turn off ALL THE LIGHTS, so it's pitch black. Do not change any settings. Take a picture. The picture should turn out the same, AND the flash wouldn't even have to work any harder. Basically, the flash is hitting the ceiling, and turning the ceiling into a large light source. THIS light source is providing all the illumination to the picture. How much flash power you need depends on the aperture, the ISO, and the distance from the light source to the subject. How dark the room is has NO affect on how much flash power is needed.

Now if you went with ISO400, 1/200,s f/5.6, and did NOT turn the flash on, the shot should be pretty dark, even with the lights on.

Now turn the flash back on, but adjust the shutter speed to 1/100s. The shot will probably look VERY close to the first two shots (you can turn the room lights back on now ) Then try 1/50s, 1/25s....Eventually you'll see the room lights "creeping" into the picture. This leads into the next paragraph...

A "flash picture" is made up of TWO distinct exposures. The "ambient" exposure if comprised of shutter speed, ISO, and f/stop. The "flash" exposure is comprised of ISO, f/stop, and flash power (and of course the distance from the light source to the subject) In the above example, the ambient exposure is essentially nil, so the picture is completely made up by the flash "components".

Once you nail using the flash to provide ALL the illumination, you can move onto more advanced topics such as balancing flash and ambient exposures.






You need to decide the CAMERA mode (Av or M, forget about using Tv/P auto modes) and the FLASH mode (Manual or E-TTL).

Indoors if the ambient light is fairly low and I'm using the flash to provide all of the illumination, I'll use M mode on the camera (and generally set the shutter speed to 1/250s to just get an ambient exposure) Outdoors where I'm using flash as fill (or indoors if it's bright, but this happens rarely) I'll use Av as I can rely on the camera to set a good general exposure WITHOUT flash, and then the flash can fill-in.

Now as for the FLASH mode, E-TTL works great if the flash is ON camera and you are constantly changing the distance between the light source and the subject. Now keep in mind what when you're bouncing, the bounce surface (ceiling or wall) actually becomes the light source. If you try to go Manual flash, you'll be adjusting the flash power for pretty much every shot, and this just isn't practical. E-TTL will get your flash power "in the ballpark"

Now once you get the flash OFF-CAMERA (on a light stand and shooting into/through an umbrella), Manual flash makes sense because although YOU can change the camera position, the light source is NOT moving (unless you move the stand of course), and as long as the subject(s) stay in the same general area, the subject-light source distance is constant. I'm talking portait/formals setups here.

Hope that helps!

09-22-2010, 08:57 AM   #17
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Basically, with flash, the FLASH exposure is solely determined by flash power (actually duration, how long the bulb is actually firing for), aperture and ISO. Ambient exposure is determined by ISO, shutter speed, and aperture (just like without any flash), so the trick is balancing the two. If I'm indoors in a smallish room (such as in someone's house), I usually just forget about ambient since the flash is powerful enough to light up the entire room (hence the 1/200s below, if the flash didn't fire, I'd have a more or less black picture) Now although you're shooting MANUAL Mode, that's only for the ambient exposure (the exposure needle in the viewfinder will blink warning you about underexposure, but ignore that). The camera's E-TTL metering will determine the needed flash output for a proper exposure.

Here's something I wrote on another forum -
"Easy" recipe for great E-TTL flash shots -
1)Point flash at ceiling/wall (to the side or behind you, experimentation is the key!)
2)Put camera in MANUAL mode on the mode dial
3)Set FEC to +1 on the flash head

4)Shoot RAW (this allows you to recover some highlights that might get blown as a result of #3 above)

5)Set ISO to 200 (to start)
6)Set shutter speed to 1/200s
7)Set f-stop to whatever DOF you want


Now if the flash runs out of "power" because of high ceilings, you can raise the ISO or open up the f-stop to compensate. Or you can slow down the shutter to bring more ambient light into the exposure (in addition to adjusting ISO/f-stop) If the ceiling is REALLY high (like in a church), you may need a reflector to throw some of the light forward (I use the Joe Demb Flip-it).

Quick and dirty outdoor fill flash tutorial -
Basically, if your subject is in shade and the background is bright (ie under a tree) or majorly backlit, fill flash is your friend. Think of those times when you got a properly exposed background, but the subject was almost pitch black.

Put camera into Av mode, metering will set the shutter speed to expose the overall shot (which in the situations that call for fill-flash will generally be the background) based on your selected aperture/ISO.
Make sure flash is set to HSS (in case your shutter speed go faster than 1/200s) and E-TTL. Fire away! The shutter speed/f-stop/ISO will expose the background, and the flash should output enough power to light up the foreground.

Now to control the background exposure, you use exposure compensation on the camera body (which would adjust the shutter speed), to adjust how much fill for the flash exposure, you use Flash exposure compensation. The trick is balancing the two (as it is with indoor work), and that comes with experience/experimentation.
09-22-2010, 07:35 PM   #18
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Shutter speed and flash

If the shutter speed only goes so high when you have the flash up it is because the camera is more knowledgeable than you! Remember, the shutter has a sync speed. This speed, somewhere between 1/180th sec and 1/100th sec., is the speed at which the focal plane shutter, quite literally a two panel curtain, is fully open. When you pop up the on-camera flash, or put on a dedicated flash, the camera knows this and reacts accordingly limit the shutter speed so you don't get exposure bands (un/under exposed areas caused by the partial closing of the focal plane shutter). If you use an older flash, then you are on your own in setting the sync speed, etc.

Shutter speed does not matter for the flash exposure (though it still matters for fill flash and ambient light) because the duration of the flash is so fast, upwards of 1/10,000 of a second in many instances. This is far faster than the fastest shutter speed you can use with our cameras. If you have a leaf blade shutter (in lens shutter) then the sync speed can be higher, sometimes up to 1/500 sec.

The camera can meter the flash exposure due to the speed of light, about 186,000 miles per second. The light leaves the flash, hits the subject, bounces back to the camera and the camera (or flash itself) determines the exposure and adjusts the flash duration accordingly. All while the shutter is open! Keep in mind, the most powerful flashes we have are only good to about 40 or 60 feet (see "inverse square law of light"), we do not have to worry about lag time from the speed of light.

Now, when we get a flash than can take a shot of the dark side of the moon from earth, then we need to be concerned (lag time, about 2 sec. each way)!

Last edited by BigDave; 09-22-2010 at 07:39 PM. Reason: typos
09-23-2010, 04:04 AM   #19
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QuoteOriginally posted by BigDave Quote

The light leaves the flash, hits the subject, bounces back to the camera and the camera (or flash itself) determines the exposure and adjusts the flash duration accordingly. All while the shutter is open!
The measuring of the pre-flash actually happens before the shutter opens, as it uses the same sensor for the ambient metering.

QuoteQuote:
Keep in mind, the most powerful flashes we have are only good to about 40 or 60 feet (see "inverse square law of light"), we do not have to worry about lag time from the speed of light.
Your "40 or 60 feet" totally ignores ISO and aperture, which are very important in Guide Number Calculations.

I was bored one night (on Halloween, wanted to scare the kiddies) so I stepped out on my porch with my camera and my external flash. f/1.4 and ISO1600 and full power was able to properly illuminate a house a good 500 feet down the street.

09-23-2010, 06:05 PM   #20
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Re: Pre-flash vs Metered flash

The TTL systems and even older on flash systems measured the actual flash, not the pre-flash. To be more accurate, flash makers also provided sensors to put on the camera, so an off camera sensor (the sensor on the flash) would not give a false light measurement.

As for the effect of ISO, yes, given a high enough ISO, the flash would have an unlimited range. However if something gets in front of your subject at a great distance, that object may become the "subject" depending on the metering system the camera/sensor employs and this new subject would be properly exposed and your intended subject under exposed. This is why Guide Number is such a tricky number, and a marketing trick if one does not pay attention to the specifics. Guide numbers must always include the ISO to be compared. Focal length of the lens is also nice. A better comparative metric needs to be decided on by the flash manufacturers to make this issue easier to compare. Most seem to settle on ISO 100, but....
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